Spintronics goes organic

Feb 25, 2004

Physicists have moved a step closer to creating a new generation of "spintronic" devices that exploit the spin of electrons as well as their charge. Jing Shi and colleagues at the University of Utah in the US have made the first organic “spin valve” - a device that changes resistance depending on the applied magnetic field. Previous spin valves were made from metals or insulators (Z Xiong et al. 2004 Nature 427 821).

A spin valve consists of a thin layer of metal or insulator sandwiched between two ferromagnetic electrodes. The spin of the electrons passing through the device can be flipped by an external magnetic field, which changes the resistance of the two ferromagnetic layers. This effect, known as magnetoresistance, has already been used to make highly sensitive magnetic-recording devices and memory chips.

Extending these spin-dependent effects to semiconductor materials has, however, proved difficult. Shi and co-workers have now built a spin valve with a 100 nanometre thick organic semiconductor made from aluminium and hydroxyquinoline. The semiconductor was sandwiched between a layer of cobalt and an alloy of lanthanum, strontium and magnesium (see figure).

To test their device, the Utah team first calculated the current that flowed through the semiconductor when the two electrodes were magnetized in the same direction – or parallel – and then in opposite directions – or anti-parallel. Shi and colleagues found that that the current increased by as much as 40% when the magnetization of the electrodes was switched from anti-parallel to parallel. This constitutes giant magnetoresistance.

At present, the device only works at low temperatures - between about –260°C to about –40°C – but Shi’s team says that the experiment is “a proof of concept that sets the stage for more practical applications”. The long-term aim is to make the device work at room temperature. The group believes that organic semiconductors have many advantages over conventional semiconductors, such as those made from silicon. They are simpler to make, are flexible and their resistance can be tuned by doping.